Final answer:
The incorrectly capitalized word is 'Small'. It should not be capitalized as it is not the first word of a sentence nor part of a title.
Step-by-step explanation:
Capitalization errors take place when the writer capitalizes a word that does not require a capital letter or when the writer does not capitalize a word the requires one. Capitalization errors hinder the reader's experience with the writing and should be avoided at all cost.
The word incorrectly capitalized in the sentence 'I'd like a Small coffee and a muffin, please.' is 'Small'. Common nouns, such as 'coffee' and 'muffin', should not be capitalized unless they are the first word of a sentence or part of a title. In this context, 'Small' is used to describe the size of the coffee and should not be capitalized. Therefore, the correct sentence should be 'I'd like a small coffee and a muffin, please.'.